In the construction of roofs, it is common practice to use flashing components around the perimeter of the roof, and particularly where the roof meets a vertical wall. Where the wall is planar, there is little difficulty in installing flashing which bridges any gap which might exist between the upstanding wall and the roof surface. Where there is a projection through an opening in the roof surface within the perimeter, for example when a vent pipe or air-conditioning conduit penetrates the roof surface, there is difficulty in providing a long-lasting seal between the projecting element and the roof surface, particularly where the projecting element is round or is of irregular contour.